The invention relates to a hydraulic system for providing pressures and volumetric flows in a dual clutch transmission, in particular for providing a first volumetric flow for cooling and lubricating a first clutch and for providing a second volumetric flow for cooling and lubricating a second clutch. In addition to cooling and lubricating the clutches, the hydraulic system also performs further tasks in dual clutch transmissions which are known from the prior art. By way of example, the hydraulic system can automatically actuate the two clutches, and gears of the dual clutch transmission are also engaged/released automatically by means of a plurality of shifting actuators.
EP 1413803 has described a method for cooling the two clutches of a dual clutch transmission in a motor vehicle. Carrying out the method requires a hydraulic system which provides a volumetric flow of oil for each clutch. The method provides for there to be a volumetric flow for the respective clutch which is dependent on an oil sump temperature, on the temperature of the oil flowing out of the respective clutch and on the power which is being transmitted by the clutches. Accordingly, a control valve or the like has to be provided for every volumetric flow in order to be able to control the volumetric flows as a function of the factors listed above.
DE 103 06 895 likewise describes a method for cooling a dual clutch of a motor vehicle; in this case, the two clutches are acted on by a single volumetric flow of a coolant together. This simplifies the structure of the corresponding hydraulic system compared to cooling with two volumetric flows. However, it is not possible for one clutch of the dual clutch transmission to be individually cooled and lubricated. Therefore, it is proposed in DE 103 06 895 that the total volumetric flow for cooling and lubricating be interrupted in particular at low temperatures, in order to reduce the drag torque caused by the coolant in the clutch and thereby to simplify the engaging of a gear. In this case, it would be sufficient to interrupt only the volumetric flow for the clutch which is assigned to the partial transmission of the dual clutch transmission in which a gear is to be engaged.
The provision of volumetric flows for in each case one clutch thereby allows targeted cooling and lubrication of the two clutches with more degrees of freedom. However, this generally leads to a greater number of hydraulic components and in particular to a greater number of expensive control valves. Moreover, when designing the hydraulic system, it needs to be ensured that at least emergency operation of the dual clutch transmission should be possible in the event of one or more of its components failing.